Mortar rounds were landing every minute in the old city area of Homs as unarmed UN observers said the Syrian army was using a drone to identify resistance fighters.

The observers could only stand and watch as the bombardment and gunfire went on for many hours.

The US says it fears the government is planning ‘another massacre’ similar to the one that left 108 men women and children dead in the village of Houla recently.

Government shelling and casualties were also reported in the province of Hama, the southern region of Daraa, the northern province of Aleppo, and suburbs of the capital Damascus and Deir el-Zour in the east.

Helicopter gunships also attacked the strategic river crossing town of Rastan, which has resisted repeated government offensives for months, activists said.

Foreign secretary William Hague said Syria is now on the verge of civil war and all options would have to be considered if peace efforts failed.

Speaking in the Commons, he said the tactics used by president Bashar al- Assad were ‘horrifyingly reminiscent of the Balkans in the 1990s’.

‘We also have reason to believe that terrorist groups affiliated to al-Qaeda have committed attacks designed to exacerbate the violence, with serious implications for international security.’

He estimated 15,000 people have now died in the uprising with 580,000 others either displaced or fled.

David Cameron will discuss the situation with other G20 leaders at the group of leading countries’ meeting in Mexico later this month.